Composite membrane suitable for use in electrochemical devices

ABSTRACT

The invention relates to novel inorganic-organic composite membranes especially useful as ionically conducting in electrochemical devices. The composites consist of a polymeric matrix, which may or may not be an ionic conductor in its unfilled form, filled with an inorganic material having a high affinity for water, capable of exchanging cations such as protons, and preferably with a high cation mobility, either on its surface or through its bulk.

This non-provisional application is a continuation of application Ser.No. 09/025,680 filed on Feb. 18, 1998, now U.S. Pat. No. 6,059,943,which claims the benefit of the Jul. 30, 1997 filing date of provisionalU.S. application Ser. No. 60/054,150.

FIELD OF THE INVENTION

This invention relates generally to the field of ionically conductingseparators. The invention particularly describes a novelinorganic-organic composite membrane especially useful as a protonconducting membrane for use in electrochemical devices, such as fuelcells.

BACKGROUND OF THE INVENTION

Conventional cation and proton conducting membranes typically comprise asheet of a homogeneous polymer, a laminated sheet of similar polymers,or a blend of polymers. A variety of polymers have been demonstrated tobe cation conductors and some of the membranes produced using thesepolymers are highlighted in Table I. All of these membranes, with theexception of the Gore Select™ membrane, are homogeneous polymers. TheGore Select™ membrane is a polymer blend.

TABLE I Polymers Used as Ion Conductors Source Name Polymer StructureDuPont Nafion ® Perfluoro side chains on a PTFE backbone Dow Perfluoroside chains on a PTFE backbone W. L. Gore Gore Select ™ Perfluoro sidechains on a PTFE backbone in a matrix Ballard Trifluorostyrene backbonewith derivatized side chains Maxdem Poly-X ™ Polyparaphenylene backboneDAIS Corp. Sulfonated side chains on a styrene- butadiene backboneAssorted Sulfonated side chains grafted to PTFE and other backbones

Two of these materials, the membranes from DuPont and Dow, haverelatively similar compositions and structures. These structures areillustrated in FIG. 1. Both of the polymers are perfluorosulfonic acids(PFSA's), which are solid organic super-acids, and both membranes areproduced as homogeneous sheets. The active ionomer component of the Goreblend is also a PFSA material.

All of those polymer materials rely on sulfonate functionalities(R—SO₃—) as the stationary counter charge for the mobile cations (H⁺,Li⁺, Na⁺, etc.), which are generally monovalent. The most commonlyproposed mechanism for this conduction, through essentially solvatedcations, is illustrated in FIG. 2, which is a schematic drawing of thecommonly proposed structure for perfluorosulfonic acid (PFSA) polymers,as typified by NAFION (a registered trademark of Dupont of WilmingtonDel.). One difficulty associated with this approach to cationconductivity is that the polymer membrane requires the presence of waterfor conductivity. As shown in FIG. 3 increasing water content increasesconductivity at all temperatures. This dependence on water is the weakpoint of membranes that rely on sulfonic acid groups for theirconductivity. As long as proton exchange membranes (PEM) membranes arekept hydrated, they function well, but when they dry out, resistancerises sharply.

The need for a PEM source of moisture besides the water generated at thecathode to maintain the amount of water in the membrane to maintainconductivity in PEM fuel cell membranes has been recognized for as longas PEM fuel cells have been known. A wide variety of methods have beendeveloped to keep membranes supplied with water. These methods typicallyrequire adding water as either vapor or liquid to the gas streamsentering the cell or adding water directly to the membrane.

There are a number of reasons that water is so easily lost from PEMs,even as it is being generated at the cathode. The vapor pressure ofwater over a saturated PEM is nearly as great as it is over pure water.This means that at a temperature of 100° C., a full atmosphere of watervapor is required to keep the membrane saturated.

The water carrying power of gaseous oxidizer streams are quitesubstantial. It is difficult to operate a fuel cell with an air flow ofless than twice the amount required to supply a stoichiometric amount ofair for oxidation of the fuel (commonly termed two-fold stoichiometry).If a fuel cell is operated at ambient pressure, operating at atemperature of 55° C. will result in the exiting air stream carrying allof the water produced by the cell at two-fold stoichiometry. Operatingat temperatures above 55° C. with the same air flow will cause a PEMmembrane to become progressively drier. Increasing the operatingpressure of the cell or stack will permit operation at highertemperatures, but the price of higher pressure is increased parasiticpower losses.

If a proton-conducting membrane could be developed with improved waterretention or a reduced dependence on free moisture for proton conductionit would be possible to operate a proton conducting membrane fuel cellwith less water, with no water, or at higher temperatures. This wouldprovide simpler, lighter fuel cell stack designs.

There is a related problem that only applies to direct methanol fuelcells (DMFC's) which is referred to as “methanol crossover.” TypicalPFSA fuel cell membranes have a higher affinity for methanol than theydo for water, as is clearly illustrated in FIG. 4. In a DMFC, thecrossover process relates to the permeation of absorbed methanol throughthe membrane from anode to cathode. In general, it has been found thatrate of methanol crossover through a PEM is proportional to the methanolconcentration in the fuel feed stream. Therefore, a proton conductingmembrane that requires less water to maintain its conductivity will alsoexhibit a reduced methanol flux.

Methanol crossover substantially impedes the performance of directmethanol fuel cells. First, methanol that crosses over represents lostfuel value and, therefore, a lower fuel efficiency. Furthermore, whenthat methanol arrives on the other side of the PEM, it is oxidized bythe cathodic electrocatatyst which depolarizes the electrode. Oxidationof methanol at the cathode increases the amount of air, or oxygen, thatthe cell or stack requires, since a molecule of methanol oxidizing onthe cathode requires the same 1½ molecules of oxygen (O₂) as one beingconsumed at the anode. Since none of the energy from this oxidation isbeing extracted as electricity, it all ends up as waste heat, increasingthe cooling load on the cell. A proton conducting membrane withsubstantially reduced methanol crossover would represent a significantimprovement in DMFC's.

Alternatives to polymer proton conductors include oxide protonconductors. A wide variety of metal oxides are proton conductors,generally in their hydrated or hydrous forms. These oxides includehydrated precious metal containing oxides, such as RuOx H₂O)_(n) and(Ru-Ti)O_(x)(H₂O), acid oxides of the heavy post transition elements,such as acidic antimony oxides and tin oxides, and the oxides of theheavier early transition metals, such as Mo, W, and Zr. Many of thesematerials are also useful as mixed oxides. Some oxides which do not fitthis description may be useful as well, such as silica (SiO₂) andalumina (Al₂O₃), although these are generally used as, or with,modifiers.

The number of metal oxides with the potential to serve as protonconductors is too large to fully discuss in detail here. This group,which can be summarized as those elements forming insoluble hydratedoxides that are not basic, includes not only known proton conductors,but oxide superacids that will furnish a multitude of free protons inthe presence of an aqueous medium. These are shown in bold in FIG. 5.Many other elements which are not included in this list may be useful inconjunction with these elements as modifiers. An example of this is theinclusion of phosphorus in the structure of Keggin ions which consistprimarily of a tungsten or molybdenum oxide framework. While thecompounds encompassed in the description above have some degree ofproton mobility, not all of those oxides have adequate proton mobilityto be useful as components in composite membranes. Some particularlyuseful examples are discussed below.

Zirconium phosphate, specifically α-zirconium phosphate, whose structureis shown in FIG. 6, is known to be an excellent proton conductor whentested as a powder at ambient temperature. Under these conditions thecompound is hydrated (Zr(HPO₄)₂(H₂O), and most of the conductivity isthe result of protons migrating over the surface of the individualcrystallites. Above 120° C. the water of hydration is lost and theconductivity drops substantially to a value representing the bulkconductivity of the solid, which increases from 1.42 μS at 200° C. to2.85 μS at 300° C. With this combination of properties, α-zirconiumphosphate is suitable for use in either low temperature (<100° C.) fuelcells, or in higher temperature (>150° C.) fuel cells.

This structure is not unique to α-zirconium phosphate. Hafnium,titanium, lead and tin all have phosphates that crystallize in thisstructure. These compounds have substantially less free volume in theirstructures than the zirconium compound, and are expected to show lowerproton mobilities.

Tungsten and molybdenum offer two groups of proton conductors. The firstof these groups are the simple, fully oxidized metals, as exemplified bytungsten trioxide (WO₃),. This compound has been the subject of muchinterest due to its electrochromic properties. This oxide can berepeatedly electrochemically reduced in the solid state, with a colorshift from light yellow to blue, and reoxidized back to the light yellowform. This property has been used to produce electrochromic windows thatcan be lightened and darkened as desired. This reaction occurs withoutany significant rearrangement of the crystal lattice. As a result,maintaining charge neutrality requires a cation (proton) to diffuse intothe structure and reside on an interstitial site. By maintaining anappropriate bias across an oxide film, a proton flux can be maintained.

The second family of tungsten and molybdenum compounds demonstrated tohave high protonic conductivity are the hetero⁺ and homo⁻ polyolybdatesand polytungstates. This description encompasses a broad range ofcompounds with widely varying compositions, all of which are based onthe fusion of groups of MO₆ (M=Mo, W) octahedra by edge or cornersharing. These ions (and they are all anions) have a generic formula of(X^(k+)M_(n)O_((3n+m)))^((2m−k)−) where k is the positive charge of theheteroatom, if any, and m is the number of unshared octahedral cornersin the structure. An example of a typical structure is illustrated inFIG. 7. The large cage in the center of the ion can host a heteroatom,such as P or As, which lowers the net charge on the ion. The exactstructure formed is a function of temperature and pH, withinterconversion between framewvorks occuring with changing conditions.

The variety of compounds in this category continues to expand, with newcompounds being synthesized and characterized regularly. Some of them,such as the (Mo₁₆V₁₄O₈₄)¹⁴⁻ ion, have very complex structures.

Compounds in this family have been demonstrated to have room temperatureproton conductivities as high as 0.17 S cm⁻¹ for H₃W₁₂PO₄₀*29H₂O and0.18 S cm⁻¹ for H₃Mo₁₂PO₄₀*29H₂O (this is over an order of magnitudegreater than the conductivity of Nafion® measured under the sameconditions). These compounds have the thermal stability to remain protonconducting above 200° C., albeit with a reduced conductivity. Not onlyare these compounds proton conductors in their own right, but whensilica gel is doped with H₃W₁₂PO₄₀*29H₂O while it is being formed fromtetraethoxysilane (TEOS) by a sol-gel reaction, then the product is anamorphous proton conductor with a conductivity that varies with theconcentration of the tungstate, which may be present at up to about 50percent by weight.

In one series of experiments, solutions of some of these acids wereimmobilized in polymer sheet matrices and the resulting electrolytemembrane used in a fuel cell to operated at room temperature whichshowed good performance. However, the significant weakness of thisapproach is that the electrolyte is present as a liquid and, therefore,is subject to displacement out of the matrix if a pressure imbalanceoccurs.

In another series of experiments the acids were used in solid form, aseither the pure acids, or in combination with a salt of the acid. Insome cases a small amount (typically 0.5%) of a resin described asethylene tetrafluoride powder was added to the acid. The addition of theresin improved the physical properties of the finished electrolyte, buteven with this addition, membranes thinner than 2 mm could not beproduced. The lower resistivity of the solid acid was not sufficient toovercome the resistive losses produced by a membrane nearly 16 times asthick as a typical polymer membrane (2 mm vs. 0.127 mm), and theresulting fuel cell exhibited poor performance. Tungsten oxides havealso been used as electrocatalyst supports, and in this role havedemonstrated an ability to enhance oxygen reduction for the platinumcatalyst on the support.

Another family of compounds that have been demonstrated to have highproton conductivity are the oxoacids of antimony. These compounds have astructure consisting of edge or corner shared SbO₆ octahedra, as shownin FIG. 8. Unshared oxygens are protonated (i.e., hydroxyls) and chargeneutrality is maintained by exchangeable external cations. In theseacids, antimony can be in either the +3 or +5 oxidation states, or amixture of the two, depending on the synthesis conditions and subsequenttreatment. The key step in the synthesis is the hydrolysis of SbCl₅,with or without hydrogen peroxide, generally carried out at 0° C. Themore oxidizing the hydrolysis conditions, the larger will be thefraction of the antimony in the +5 oxidation state in the final product,and with a sufficiently oxidizing hydrolysis solution it is possible toobtain acids with all of the antimony in the +5 state. The acidprecipitates as an insoluble white powder having a pyrochlore-typeframework structure (based on cubic symmetry). The powder is thoroughlywashed and dried at room temperature before further use.

Antimonic acids are dehydrated on heating in dry air, with most of thewater lost at around 140° C. As long as the material is not heated above200° C. it will reabsorb water from air, even under normal roomconditions, and return to its original weight. Heating to temperaturesabove 300° C. lead to deoxygenation, with the Sb⁺⁵ present reverting toSb⁺³.

Thin films of antimonic acid have been produced on conductive surfacesby electrophoretically depositing fine particles suspended in a solutionof ammonium hydroxide in acetone. Although the resulting layers wereshown by SEM to be smooth, no information was given on whether or notthey were pore free, a requirement for this application.

Like tungsten and molybdenum, tantalum and niobium form highly chargedcomplex polyanions, as illustrated in FIG. 9. These materials are alsofacile cation exchangers capable of proton conduction and subject toirreversible dehydration if heated above 100° C.

These families of inorganic ion exchangers have significant differences,but they also have three common features that make them candidates foruse as proton conducting electrolytes in fuel cells. First, they allhave easily exchangeable protons. Second, they all have open frameworkstructures with channels to provide low resistance paths for the mobileprotons to move along. Third, they all retain their proton conductivityat temperatures in excess of 200° C., and in most cases, in excess of300° C. This last characteristic would appear to make it possible to usethese compounds in fuel cells operating at slightly elevatedtemperatures as well as at the same low temperatures (<100° C.) whereconventional PEM fuel cells are used. Unfortunately, all of these oxideproton conductors are ceramic materials which are difficult to fabricateinto thin, pin hole free, films.

There are other inorganic compounds, with significantly differentstructures, which also offer a high degree of proton mobility. Theseinorganic compounds include solid superacids and oxides with highlyhydrated surfaces as shown simplistically in FIG. 10. In both cases, theproton conductivity comes from protons diffusing over the surface ofindividual crystallites, or particles in the case of amorphousmaterials. This effect has already been described for fully hydratedα-zirconium phosphate. The Grothaus proton hopping occurring here is thesame process that is presumed to account for the proton conductivity ofPFSA membranes, polyphosphoric acid, and tungstic acid, as illustratedin FIG. 11.

Hydrated ruthenium oxides are one of the materials known to be capableof supporting a significant ionic current through the surface protonhopping mechanism described above. However, pure RuOx (H₂O)_(n) wouldnot be acceptable for use in electrolyte membranes since this compoundis a metallic conductor. As such, it would electrically short circuitany cell in which it is used.

Ruthenium oxide “stuffed” Nafion® membranes have been tested aselectrolyte membranes in direct methanol fuel cells and weredemonstrated to reduce methanol crossover. Unfortunately, in thisincarnation they were also found to reduce proton conductivitysignificantly.

A recently reported aerogel synthesis has been demonstrated to beparticularly effective in generating proton conducting materials,largely because the products of this reaction have very high surfaceareas with a high degree of hydroxyl terminations and good electricalseparation of local RuO_(x) domains. (Ru_(0.32)Ti_(0.68))O₂ is a mixedconductor with both electrons and protors acting as charge carriers, andflowing in opposite directions. When normally synthesized as a bulkmaterial, the majority of the current is carried by electrons. When thematerial is synthesized as an aerogel, with a greatly increased surfacearea, the majority of the charge is carried by protons. This is a cleardemonstration of the surface protonic conductivity of RuO and a clearroute to a way of utilizing it. The key to the aerogel process iskeeping the widely dispersed sol-gel network, which is produced by thehydrolysis of a relatively dilute solution of metal alkoxides, separatedas the solvent is removed. A similar effect can be harnessed in theproduction of membranes, as described in a later section of thisdisclosure.

Sulfonated zirconia is an amorphous solid super acid that has recentlyreceived significant attention as an acid catalyst primarily for use inhydrocarbon conversions and as an acid support for other catalysts.Titanium oxides, and titanium-aluminum oxides, have been shown to havesimilar properties, but this discussion will focus on the better knownzirconia compounds.

These materials are generally viewed as amorphous metal oxides withsulfate groups attached to their surface. They are produced by a varietyof routes. The classical method is precipitation of amorphous Zr(OH)₄ bytreating an aqueous solution of a zirconium salt with a base followed bysulfonation of the gel with either sulfur acid or ammonium sulfate. Theamorphous Zr(OH)₄ can also be produced by a sol-gel method, are sulfatedin the same way. Both of these methods are essentially two-stepsyntheses. Higher surface area materials can be produced by the directreaction of sulfuric acid with the alkoxide precursor. The catalyst isactivated before use by calcination at temperatures between 400° and650° C. Although these materials are strong Bronsted acids, like PFSAmaterials, they require water for the formation of free protons.

Solids with similar properties can also be produced with alumina (Al₂O₃)serving in place of zirconia. These materials are produced by combininga salt, such as Li₂SO₄ or RbNO₃, with the corresponding aluminum saltand sintering the mixture to convert the aluminum salt to an aluminamatrix. The guest salt remains relatively unchanged. These materials canbe pressed to form tablets about 1-2 mm thick, which were tested as fuelcell electrolytes. When operated at 400° C. they were found to producepromising results, with single cell potentials as high as 0.75 Vobserved at current densities of 200 mA/cm². The conductivity wasattributed to protons moving along sites formed by the salt in thealumina matrix based on IR evidence of H—SO₄ coordination in the lithiumcontaining electrolyte. However, because of the high temperaturerequired for conductivity, these materials are not considered promisingfor use in a polymer bonded system.

All of the oxides described above are potentially useful as protonconductors, if they could se fabricated into sufficiently thin sheetsthat the conductivity would be similar to conventional polymericmembranes. The inability to produce thin sheets is a key weakness ofmaterials produced by the approach or method used by Nakamora et al.(U.S. Pat. No. 4,024,036.)

In addition to inorganic cation conductors, inorganic-organic compositemembranes are potentially useful for electrochemical applications. PFSAmembranes, such as Nafion®, have been filled with 12-phosphotungsticacid (H₃W₁₂PO₄₀), an inorganic proton conductor. These membranes havebeen demonstrated to have better water retention and, consequently,better conductivity at temperatures above 100° C. than the samemembranes in their unfilled form. The goal was to develop membranes forPEM fuel cells that could be operated at elevated temperatures toameliorate the problem of CO poisoning for anode electrocatalysts. Theaddition of 12-phosphotungstic acid to the polymer electrolyte permittedoperation at temperatures up to 120° C., but no evidence was shown forimproved CO tolerance.

In U.S. Pat. No. 5,523.181, Stonehart et al. describe a compositemembrane useful for PEM fuel cells consisting of high surface areasilica, preferably in the form of fibers, as a filler with a variety ofpolymers capable of exchanging cations with solutions as the matrix.These membranes are produced by suspending the inorganic phase in asolvent appropriate for the dissolution of the polymer and blending thesuspension with a solution of the polymer in the same solvent. Membranesare formed by evaporating the solvent in a controlled manner to producea thin film of the composite. The silica is selected to maximize itsaffinity for water and ability to retain water. They demonstrate reducedelectrical resistance in fuel cells operating under conditions of lowhumidification. The improved performance is attributed to improved waterretention by the silica, and improved back diffusion of water from thecathode to the anode along the silica fibers with the back diffusingwater replacing water removed by electroosmotic transport. They have notattributed any contribution to the overall proton conductivity to thesilica.

In U.S. Pat. No. 5,512,263, McIntyre describes a composite membraneproduced using an ionically conductive polymer together with anelectrically conductive filler phase. This membrane permits theconstruction of an internally shorted fuel cell, which is described asuseful for the synthesis of hydrogen peroxide. Since all of theelectrical current flows internally within the membrane, there is noexternal electrical control or monitoring of the reaction. This lack ofcontrol may contribute to the relatively low efficiency of theirprocess.

In U.S. Pat. 5,682,261, Takada et al. disclose a three phase system forproducing a composite membrane. A Brønsted acid, typically a strongmineral acid is adsorbed onto the surface of finely divided silica andthis mixture is combined with a thermoplastic binder to produce a protonconducting membrane. In this membrane the primary conductivity is due tofree protons in the acid. This membrane has been found to be useful asan ion conductor for electrochromic windows and for fuel cells.

In U.S. Pat. No. 5,334,292, Rajeshwar et al. describe a compositeconsisting of an electron conducting polymer (as opposed to an ionconducting electrolyte) and catalytically active metal particles. Thepolymers they use are polypyrrole and polyanaline which are polymerizedelectrochemically on a conductive surface. This composite is describedas being useful as a supported electrocatalyst where it is desirable tosuspend precious (e.g., Pt, Pd, Ag, Ru, etc.) electrocatalyticallyactive particles in an inexpensive conductive matrix to minimize theamount of precious metal used.

Inorganic-organic composite membranes may also be useful for a varietyof other applications. These composites may include a Nafion® matrix anda semiconductor filler, where the semiconductors generally selected arethose known to show activity for carrying out photocatalytic reactions,such as CdS, CdSe, FeS₂, ZnS, TiO₂, and Fe₂O₃. The composites producedare useful for carrying out reactions such as the photocatalyticdecomposition and oxidation of organic compounds and even the fixationof nitrogen.

In their article entitled “Nafion/ORMOSIL Hybrids via in Situ Sol-GelReactions. 3. Pyrene Fluorescence Probe Investigations of NanoscaleEnvironment,” (Chemistry of Materials, 9, 36-44, (1997), Mauritz et atdescribe PFSA-silica composites by the hydrolysis of tetraethoxysilane(TEOS) inside the polymer matrix. The inorganic-organic ratio can bevaried over a wide range, as can the properties of the inorganic phase,permitting the properties of the final composite to be tailored forspecific applications. These composite materials have been demonstratedto have improved selectivity for gas separation when compared to theunfilled polymer. Mauritz et al. have also demonstrated the ability toproduce nanophase composites with TiO₂, titaniasilicate, andaluminasilicate inorganic phases.

A number of authors have described PFSA membranes filled with transitionmetal complexes. Compounds used in this way have included complexes ofruthenium, rhodium, palladium, silver, rhenium, iron, and manganese. Inthese composites, the polymer serves to immobilize and stabilize theionic or molecular species, which continues to exist in essentially thesame form as it does in solution. These complexes usually serve ascatalysts, with the transition metal species serving the same functionas it does when used as a homogeneous catalyst, but with improvedcatalyst life and recoverability. In the case of silver, the filledmembrane was found to have improved selectivity for separation of dienesfrom monoenes. While these materials are sometimes described ascomposites, they are more accurately described as immobilizedhomogeneous catalysts with a polymer as the support.

Therefore, there is a need for ionically conducting materials orcomposites exhibiting high cation conductivity and reduced dependence onwater. It would be desirable if these materials or composites performedwell not only at temperatures below about 100° C., but also above about150° C. or more. It would also be desirable if the materials orcomposites were suitable for use as PEMs in fuel cells, particularly infuel cells using reformate fuels which may contain carbon monoxide.

SUMMARY OF THE INVENTION

The present invention provides a cation-conducting composite membranecomprising an oxidation resistant polymeric matrix filled with inorganicoxide particles forming a connected network extending from one face ofthe membrane to another face of the membrane. In many applications, thecations will comprise protons. The inorganic oxide particles maycomprise a hydrated metal oxide, preferably wherein the metal isselected from molybdenum, tungsten, ziroconium and mixtures thereof, andmost preferably wherein the inorganic oxide particles are selected fromheteropolytungstates, heteropolymolybdates, zirconium phosphates, andmixtures thereof. The polymeric matrix is preferably a synthetic organicpolymer having a melting point greater than about 300° C., such aspolymers selected from fully halogenated polymers, partially halogenatedpolymers, and mixtures thereof. The preferred synthetic organic polymersare selected from perfluorosulphonic acid, polytetrafluoroethylene,perfluoroalkoxy derivative of PTFE, polysulfone, polymethylmethacrylate,silicone rubber, sulfonated strene-butadiene copolymers,polychlorotrifluoroethylene (PCTFE), perfluoroethylene-propylenecopolymer (FEP), ethylene-chlorotrifluoroethylene copolymer (ECTFE),polyvinylidenefluoride (PVDF), copolymers of polyvinylidenefluoride withhexafluoropropene and tetrafluoroethylene, copolymers of ethylene andtetrafluoroethylene (ETFE), polyvinyl chloride, and mixtures thereof,with the most preferred halogenated polymers being the fluorinatedpolymers and blends of these polymers.

The cation-conducting composite membranes of the present invention maybe made by several processes of impregnating an inorganic oxide protonconductor into the pores of a porous polymer matrix. One method includesimpregnating the precursor to an oxide proton conductor into the poresof a porous polymer matrix; and then converting the precursor materialinto the desired proton conducting oxide. Another method includesprecipitating a mixture of the ion conducting oxide and a polymer matrixfrom solution and pressing the precipitate into a membrane. Yet anothermethod includes converting a soluble precursor to a proton conductivemetal oxide to the insoluble oxide in an aqueous solution containing anemulsion or suspension of a polymer and simultaneously precipitating thepolymer and the metal oxide. A further method includes providing asuspension of a suitable inorganic proton conductor in a solution,forming a matrix polymer within the solution, separating the solids fromthe solution, and pressing the solids into a composite membrane. Onemethod includes producing a suspension containing both the matrixpolymer and the oxide proton conductor, filtering the matrix polymer andthe oxide proton conductor onto a removable filter element, removing thefiltered mass from the removable filter element, and pressing thefiltered mass to form composite membrane. The pressing may occur withheat and may occur between two rollers. The pressed membrane shouldexhibit good mechanical properties, such as flexibility, and beessentially gas impermeable. The filling of the inorganic matrix may beconducted so that the distribution of the oxide in the matrix may beeither homogeneous or heterogeneous. In a homogeneous filling, allportions of the matrix may comprise essentially equivalent quantities ofthe oxide, while in a heterogeneous filling, some portions of the matrixmay comprise more oxide than other portions of the matrix. Inheterogenous fillings, it is possible that some portions of the matrixmay comprise little oxide or no oxide at all. Finally, another methodincludes using a porous polymer matrix as a filter element, and fillingthe pores in the porous polymer matrix with an oxide proton conductor byfiltering a suspension of the oxide into it.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the above recited features and advantages of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference to theembodiments thereof which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic diagram of the structure of perfluorosulfonic acid(PFSA) polymers, as used in proton exchange membranes such as Nafion®115 (n≈6.5 and m=1) and the Dow Chemical membrane (n≈6 and m=0).

FIG. 2 is a schematic diagram illustrating the key features of aperfluorosulfonic acid (PFSA) membrane, i.e., a PTFE-like backbone phaseseparated from an ionic region that closely resembles an aqueouselectrolyte by an interphase or side chain region.

FIG. 3 is a graph of the conductivity of Nafion® as a function ofmoisture content measured at four temperatures.

FIG. 4 is a bar graph showing the relative degree of swelling of Nafion®115 in water, methanol, and selected water-methanol solutions. Swellingis defined as the gain in weight on exposure to the solvent for a fullydried specimen. All specimens were soaked in solvents at roomtemperature then equilibrated at 50° C. for 70 minutes.

FIG. 5 is a section from the periodic table of the elements with thoseelements forming hydrous oxides potentially useful for forming compositemembranes indicated in bold face type.

FIG. 6 is a diagram of the layered structure of α-zirconium phosphate.

FIG. 7 is a diagram of the structure of a typical heteropolymolybdate,(X^(n+)Mo₁₂O₄₀)^((8−n)−), a β-Keggin-type ion.

FIGS. 8A and 8B are diagrams of the structure of two antimonic acidpolymers, H₃Sb₃O₄(OH)₁₀ and H₅Sb₅O₄(OH)₂₂.

FIG. 9 is a diagram of the structure of the M₆O₁₉ ⁸⁻(M=Nb or Ta) anion.

FIG. 10 is a schematic diagram of amorphous oxide particles with highlyhydroxylated surfaces capable of supporting proton conduction over thesurface.

FIG. 11 is a schematic diagram of the Grothaus proton hopping mechanismfor conductivity in an aqueous acid environment.

FIG. 12 is a cross-section view of an ion conducting membrane producedaccording to the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a composite membrane consisting of an inorganiccomponent, active for the conduction of protons or other cations, boundtogether by a polymeric binder phase, which may, or may not, be an ionicconductor. This membrane requires a sufficient fraction of the inorganicphase to insure connectivity from one surface of the membrane to theopposing surface. It also requires a sufficient fraction of the polymerphase to produce good barrier properties to prevent mass transportthrough the membrane and to impart a degree of flexibility to themembrane.

The inorganic component forms a connected network, shown in FIG. 12,penetrating from one face of the membrane to the other face of themembrane to permit ionic conduction from face to face. As can be seen inFIG. 12, it is likely that not every particle will be in contact withother particles, or that every chain of particles will extend completelythrough the membrane. In order to insure as many particle-to-particlecontacts as possible, it is important that the volume fraction of theinorganic phase be as large as possible, without sacrificing the barrierproperties of the membrane.

The oxide components suitable for use in the composite membranes of thepresent invention are described in the background section above. Thepolymer matrix used is equally important, and there are a variety ofmaterials available to fill this role. Some of these, together with someof their advantages, are summarized in Table II.

TABLE II Polymers Useful for Forming Composite Membranes Polymer NameAdvantages Perfluorosulfonic acid (PFSA) Nafion ® Resistant to oxidationand a cation conductor. Polytetrafluoroethylene Teflon ® Resistant tooxidation (PTFE) Perfluoroalkoxy derivative of Resistant to oxidationPTFE (PFA) Polysulfone Good temperature resistance PolymethymethacrylateEasily formed, inexpensive (PMMA) Silicone rubber Easily fabricatedPolyvinyl chloride (PVC) Thermoplastically formed

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Perfluorinated materials, such as PTFE and PFA, have several distinctadvantages for this application. The primary advantage of these is theirnear total resistance to oxidation. Some electrochemical applicationsinvolve relatively mild conditions, but one of the largest potentialapplications, PEM fuel cells exposes the electrolyte membrane toextremely oxidizing conditions. Not only is one side of the membraneexposed to air at elevated temperatures, but the fuel cell reactionsthemselves produce trace levels of hydrogen peroxide and peroxylradicals. Both of these are extremely powerful oxidizers which readilyattack hydrocarbons and partially halogenated polymers.

A second advantage is a relatively high melting point. With meltingpoints in excess of 300° C., these materials are suitable for service attemperatures up to 250° C.

PFSA polymers are perfluorinated for oxidation resistance, and are alsoion conducting polymers. These have already been demonstrated to beeffective matrices for the formation of composites. These polymers alsohave a strong affinity for water and consequently, undergo a significantchange in size with changing water activity. Not only do they changesize with water activity, but, as shown in FIG. 3, when water activitydeclines, so does the polymer's ionic conductivity. Even at moderatelyelevated temperatures, just above 100° C., a water pressure in excess ofone atmosphere is required to keep the polymer ionically conductive.

Polysulfone is a thermoplastic with excellent heat resistance and a highheat deflection temperature (150° C.). Composite membranes made with apolysulfone matrix have higher maximum service temperatures than thosewith PFSA matrices and are less subject to creep than those made withPTFE matrices.

Other polymers, such as polymethylmethacrylate, silicone rubber, andpolyvinyl chloride, are also useful, with each having its own usefulproperties. Regardless of which polymer is used as the matrix, it isimportant that the polymer remain flexible under fuel cell operatingconditions, and that any size change that occurs with changingconditions be relatively small.

There are other polymers not listed above which could be used in thesame manner as those described here. The selection and use of otherpolymers as the binder phase to produce membranes in accordance with thepresent invention may be readily recognized by one skilled in the art.

The composite membranes of the present invention may be made in avariety of ways. These include, but are not limited to, the following:

1) Impregnating an oxide proton conductor into the pores of a porouspolymer matrix. This is the most direct method.

2) Impregnating the precursor to an oxide proton conductor into thepores of a porous polymer matrix and then converting the precursormaterial into the desired proton conducting oxide. An example of thismethod is the impregnation of a porous, expanded sheet of polytetrafluoro ethylene (PTFE) sold under the tradename GORETEX (W.L.Associates, Inc., Elkton Md.) or the tradename TETRATEX (Tetratec,Seasterville, Pa.) having pores in the range of 0.005-3 μm in diameterwith a concentrated metal alkoxide solution, followed by hydrolysis toproduce the desired hydrated oxide and drying to remove excess water. Ifthe solubility or solubilities of the metal alkoxides are insufficientfor a single deposition cycle to deposit enough metal oxide to supportionic conductivity or to remind the membrane essentially impermeable togas flow, the metal oxide content can be increased after drying byrepeating the deposition and hydrolysis process.

3) Precipitating a mixture of the ion conducting oxide and the polymerbinder from an aqueous solution and fabricating the resulting productinto a membrane.

4) Converting a precursor to a proton conductive metal oxide to theoxide in aqueous solution containing an emulsion or suspension of thepolymer and simultaneously precipitating the polymer along with theoxide.

5) Carrying out the solution polymerization of a polymer suitable foruse as the binder phase of the composite membrane in a suspension of asuitable inorganic proton conductor, separating the solids from thesolution and producing a composite membrane.

6) Producing a suspension containing both the desired polymer binder andthe oxide proton conductor, then filtering the suspension onto aremovable filter element to produce the desired membrane.

7) Using a porous polymer filter element as the polymer phase, andfilling the pores in the filter with the desired oxide proton conductorby filtering a suspension of the oxide into it. An example of this isthe vacuum filtration of an amorphous zirconium phosphate gel into aporous, expanded polytetra fluoro ethylene (PTFE) filter elementfollowed by crystallization of α-zirconium phosphate by refluxing themembrane in phosphoric acid.

8) Filling the pores of a porous polymer membrane with a concentratedsolution of the precursor to the desired oxide, with a reservoir ofprecursor solution on one side and precipitating the oxide phase in thepores of the filter by placing an appropriate reactive solution on theother side, with the reaction occurring inside the membrane as thecomponents diffuse together. An example of this is the precipitation ofa hydrous oxide from an acidic metal salt solution by raising the pHthrough the use of a basic solution.

There are two primary methods for synthesizing the oxide protonconductors described here. The most common route is base hydrolysis ofsoluble metal chlorides, metal nitrates or other metal salt solutions.Most transition metal chlorides and nitrates are soluble in acid. Whenthe solution pH is raised by the addition of base, the metalprecipitates as hydroxide. Further processing, by either calcination orhydrothermal treatment, converts the hydroxide to the oxide.

The second route is the sol-gel route. In this process a metal hydroxidegel is formed by slowly adding small amounts of water to a solution ofthe metal alkyl or alkoxide in a hydrocarbon (or other unreactiveorganic) solvent. The water reacts to form alkanes (from alkyls) oralcohols (from alkoxides) and a dispersed metal hydroxide gel. This gelcan be consolidated by aging or converted to an oxide by hydrothermaltreatment or calcination.

There are several important considerations when processing protonconducting metal oxides that apply to any approach for membranefabrication. The first of these is temperature sensitivity. Most of theinorganic oxides discussed here lose all, or part, of their protonconductivity if heated to too high a temperature in the absence ofwater. The details of how a number of these compounds behave when heatedwere discussed in the background section. Both Zr(HPO₄)₂.H₂O andH₃W₁₂PO₄₀.29H₂O are proton conductors at elevated temperatures (>200°C.) as well at lower temperatures (˜60-100° C.), but both are betterconductors at low temperatures than at high temperatures and both, ifannealed at temperatures above 300° C., will be transformed irreversiblyto the lower conductivity form.

The second broad consideration is cleanliness in the sense that theoxide needs to be as free as possible of free anions, especially halidessuch as chlorides. Halides are known to poison precious metal catalysts,such as platinum which is frequently used in fuel cells. Even smallamounts of free anions can cause severe problems, since the sameelectric field that causes protons to migrate from anode to cathode willcause any free anions present to migrate toward the cathode potentiallypoisoning the electrocatalysts.

The following examples show the function of this invention and some ofits preferred embodiments.

EXAMPLE 1 This Example Illustrates the Preparation of a CompositeMembrane

A porous polytetrafluoroethylene (PTFE) filter is placed in a filtrationfunnel, a suspension of gelatinous zirconium phosphate produced by therapid addition of a solution of Zr(NO₃)₄. 5H₂O to 85% phosphoric acid isplaced in the funnel, and a vacuum applied to the outlet of the funnel.Fluid moves through the pores in the filter, but the gelatinouszirconium phosphate is retained, rapidly filling the pores and bringingthe filtration to a stop after about 10 minutes. At the point the vacuumis released, that portion of the original suspension which has not yetpassed into the filter is decanted, and the filter transferred to aboiling flask containing 85% phosphoric acid. A condenser is placed ontop of the flask and the flask heated until the acid reaches reflux.

The heating is continued for seven days to crystallize the gel intoα-Zr(HPO₄)₂.H₂O. After the flask cools, the acid is decanted and themembrane is removed. Since the crystalline phosphate occupies lessvolume than the amorphous gel, there is now pore volume open in themembrane. To insure the complete filling of the pores, the filtrationstep is repeated, followed by the reflux step. The resulting membrane iswashed in deionized water to remove any free phosphoric acid.

The composite membrane fabricated above may be submitted to a Gurley airflow test to determine its gas permeability properties. The Gurley airflow test measures the time in seconds for 100 mL of air to flow througha one square inch sample at 4.88 inches water pressure. The sample ismeasured in a Gurley Densometer (ASTM 0276-58). The placed between theclamp plates. The cylinder is then dropped gently. The automatic timer(or stopwatch) is used to record the time (seconds) required for aspecific volume recited above to be displaced by the cylinder. This timeis the Gurley number.

EXAMPLE 2 This Example Illustrates the Use of a Composite Membrane

A membrane produced by the method described in Example 1 is placedbetween two standard gas diffusion electrodes, the surfaces of whichhave been catalyzed with carbon (Vulcan XC-72R) supported platinum (30wt %) at a loading of 2 mg Pt/cm² and the entire assembly placed in astandard fuel cell testing apparatus. Hydrogen is supplied to oneelectrode (the anode) and air to the opposite electrode (the cathode)and the resulting cell operated as a fuel cell for the generation ofelectricity.

While the foregoing is directed to the preferred embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims which follow.

What is claimed is:
 1. A method of making a cation-conducting compositemembrane comprising the step of impregnating an inorganic oxide protonconductor into the pores of a porous polymer matrix.
 2. The method ofclaim 1, wherein the cation-conducting composite membrane conductsmonovalent charged cations.
 3. The method of claim 2, wherein themonovalent charged cations are protons and alkali metal cations.
 4. Themethod of claim 1, wherein the inorganic oxide proton conductor ischaracterized in that it possesses easily exchangeable protons, has anopen framework structure with channels, and retains proton conductivityat temperatures in excess of 200° C.
 5. The method of claim 1, whereinthe inorganic oxide proton conductor comprises insoluble hydrated orhydrous metal oxides selected from the group consisting of hydratedprecious metal containing oxides, acid oxides of the heavy posttransition elements, oxides of the heavier early transition metals,oxide superacids, phosphates of zirconium, phosphates of hafnium,phosphates of titanium, phosphates of lead, phosphates of tin, tungstentrioxide, molybdenum trioxide, hetero-polymolybdates, andhomo-polymolybdates, hetero-polytungstates, homo-polytungstates,hetero-polytantalates, homo-polytantalates, hetero-polyniobates,homo-polyniobates, oxoacids of antimony, and mixtures thereof.
 6. Themethod of claim 5, wherein the hydrated precious metal containing oxidesare selected from the group consisting of RuO_(x)(H₂O)_(n) and(Ru-Ti)O_(x) (H₂O)_(n).
 7. The method of claim 5, wherein the acidoxides of the heavy post transition elements are selected from the groupconsisting of acidic antimony oxides and acidic tin oxides.
 8. Themethod of claim 5, wherein the oxides of the heavier early transitionmetals comprise metals selected from the group consisting of molybdenum,tungsten and zirconium.
 9. The method of claim 5, wherein the oxidesuperacids are selected from the group consisting of sulfonatedzirconia, sulfonated titanium oxide, and sulfonated titanium-aluminumoxides.
 10. The method of claim 1, wherein the inorganic protonconductor comprises a hydrated or hydrous mixed oxide.
 11. The method ofclaim 1, wherein the inorganic proton conductor comprises elementsforming insoluble hydrated oxides that are not basic.
 12. The method ofclaim 1, wherein the inorganic proton conductor comprises oxideparticles that form a connected network extending from one face of themembrane to another face of the membrane.
 13. The method of claim 1,wherein the porous polymer matrix is a synthetic organic polymer havinga melting point greater than about 300° C.
 14. The method of claim 13,wherein the synthetic organic polymer comprises a fully halogenatedpolymer, a partially halogenated polymer, and mixtures thereof.
 15. Themethod of claim 14, wherein the most preferred halogenated polymers arefluorinated polymers and blends of these polymers.
 16. The method ofclaim 13, wherein the synthetic organic polymer is resistant tooxidation.
 17. The method of claim 1, wherein the porous polymer matrixcomprises perfluorosulphonic acid, polytetrafluoroethylene,perfluroalkoxy derivative of PTFE, polysulfone, polymethylmethacrylate,silicone rubber, sulfonated styrenebutadiene copolymers,polychlorotrifluoroethylene (PCTFE), perfluoroethylenepropylenecopolymer (FEP), ethylene-chlorotrifluoroethylene copolymer (ECTFE),polyvinylidenefluoride (PVDF), copolymers of polyvinylidenefluoride withhexa fluoropropene and tetrafluoroethylene, copolymers of ethylene andtetrafluoroethylene (ETFE), polyvinyl chloride, and mixtures thereof.18. The method of claim 1, wherein the cation-conducting compositemembrane comprises a sufficient fraction of the inorganic oxide protonconductor to insure oxide particle-to-oxide particle connectivity fromone surface of the membrane to the opposing surface.
 19. The method ofclaim 1, wherein the polymer matrix comprises a perfluorosulphonic acid.20. The method of claim 1, wherein the inorganic oxide proton conductoris synthesized by means of base hydrolysis of metal salt solutions. 21.The method of claim 20, wherein the metal salt solution comprises ametal chloride, a metal nitrate, or mixtures thereof.
 22. The method ofclaim 12, wherein the inorganic oxide proton conductor is synthesized bymeans of a sol-gel route.
 23. The method of claim 1, wherein the polymermatrix comprises porous polytetrafluoroethylene.
 24. The method of claim23, wherein the porous polytetrafluoroethyline comprises an expandedsheet of polytetrafluoroethylene.
 25. A method of making acation-conducting composite membrane comprising the steps of: (a)impregnating the precursor to an oxide proton conductor into the poresof a porous polymer matrix; and then (1) converting the precursormaterial into the desired proton conducting oxide.
 26. A method ofmaking a cation-conducting composite membrane comprising the steps of:(a) precipitating a mixture of a cation conducting oxide and a polymermatrix from solution; and (b) fabricating the resulting product into amembrane.
 27. A method of making a cation-conducting composite membranecomprising the steps of: (a) converting a soluble precursor to a protonconductive metal oxide to the insoluble oxide in an aqueous solutioncontaining an emulsion or suspension of a polymer; and (b)simultaneously precipitating the polymer and the metal oxide.
 28. Themethod of claim 27, wherein the polymer comprises a polymeric binderphase.
 29. The method of claim 27, wherein the polymer comprises anionic conductor.
 30. A method of making a cation-conducting compositemembrane comprising the steps of: (a) providing a suspension of aninorganic proton conductor in a solution; (b) polymerizing a polymerwithin the solution; (c) separating the solids from the solution; (d)pressing the solids into a composite membrane, wherein the polymer isthe binder phase of the composite membrane.
 31. The method of claim 30wherein pressing the solids into a composite membrane comprises rolling.32. The method of claim 30 wherein the pressing is carried out in thepresence of heat.
 33. A method of making a cation-conducting compositemembrane comprising the steps of: (a) producing a suspension containingboth the polymer matrix and the oxide proton conductor; and (b)filtering the polymer matrix and the oxide proton conductor onto aremovable filter element; (c) removing the filtered mass from theremovable filter element; and (d) pressing the filtered mass to form thecomposite membrane.
 34. A method of making a cation-conducting compositemembrane comprising the steps of: (a) using a porous polymer matrix as afilter element; and (b) filling the pores in the porous polymer matrixwith an oxide proton conductor by filtering a suspension of the oxideinto it.
 35. The method of claim 34 further comprising pressing thecomposite membrane.
 36. The method of claim 35 wherein the pressingcomprises communicating the composite membrane with a heat source. 37.The method of claim 36, wherein the cation-conducting composite membraneis flexible and essentially gas impermeable.
 38. The method of claim 34,where in the step of filling the pores in the porous polymer matrix withan oxide proton conductor yields either a homogeneous or heterogeneousdistribution of oxide particles.
 39. The method of claim 34, wherein theoxide proton conductor is characterized in that it possesses easilyexchangeable protons, has an open framework structure with channels, andretains proton conductivity at temperatures in excess of 200° C.
 40. Themethod of claim 34, wherein the oxide proton conductor comprisesinsoluble hydrated or hydrous metal oxides selected from the groupconsisting of hydrated precious metal containing oxides, acid oxides ofthe heavy post transition elements, oxides of the heavier earlytransition metals, oxide superacids, phosphates of zirconium, phosphatesof hafnium, phosphates of titanium, phosphates of lead, phosphates oftin, tungsten trioxide, molybdenum trioxide, hetero-polymolybdates, andhomo-polymolybdates, hetero-polytungstates, homo-polytungstates,hetero-polytantalates, homo-polytantalates, hetero-polyniobates,homo-polyniobates, oxoacids of antimony, and mixtures thereof.
 41. Themethod of claim 40, wherein the hydrated precious metal containingoxides are selected from the group consisting of RuO_(x)(H₂O)_(n) and(Ru-Ti)O_(x) (H₂O)_(n).
 42. The method of claim 40, wherein the acidoxides of the heavy post transition elements are selected from the groupconsisting of acidic antimony oxides and acidic tin oxides.
 43. Themethod of claim 40, wherein the oxides of the heavier early transitionmetals comprise metals selected from the group consisting of molybdenum,tungsten and zirconium.
 44. The method of claim 40, wherein the oxidesuperacids are selected from the group consisting of sulfonatedzirconia, sulfonated titanium oxide, and sulfonated titanium-aluminumoxides.
 45. The method of claim 34, wherein the oxide proton conductorcomprises a hydrated or hydrous mixed oxide.
 46. The method of claim 34,wherein the oxide proton conductor comprises elements forming insolublehydrated oxides that are not basic.
 47. The method of claim 34, whereinthe oxide proton conductor comprises oxide particles that form aconnected network extending from one face of the membrane to anotherface of the membrane.
 48. The method of claim 34, wherein the porouspolymer matrix is a synthetic organic polymer having a melting pointgreater than about 300° C.
 49. The method of claim 48, wherein thesynthetic organic polymer comprises a fully halogenated polymer, apartially halogenated polymer, and mixtures thereof.
 50. The method ofclaim 49, wherein the most preferred halogenated polymers arefluorinated polymers and blends of these polymers.
 51. The method ofclaim 34, wherein the porous polymer matrix comprises perfluorosulphonicacid, polytetrafluoroethylene, perfluroalkoxy derivative of PTFE,polysulfone, polymethylmethacrylate, silicone rubber, sulfonatedstyrenebutadiene copolymers, polychlorotrifluoroethylene (PCTFE),perfluoroethylenepropylene copolymer (FEP),ethylene-chlorotrifluoroethylene copolymer (ECTFE),polyvinylidenefluoride (PVDF), copolymers of polyvinylidenefluoride withhexa fluoropropene and tetrafluoroethylene, copolymers of ethylene andtetrafluoroethylene (ETFE), polyvinyl chloride, and mixtures thereof.52. A method of making a cation-conducting composite membrane comprisingthe steps of: (a) placing the first side of a porous polymer membrane incontact with a concentrated solution of a precursor to an oxide protonconductor; (b) placing the second side of the porous polymer membrane incontact with a reactive solution comprising a component that reacts withthe precursor component; (c) allowing the precursor and reactivecomponents to diffuse into the bulk of the porous polymer membrane fromthe first and second sides, respectively; and (d) reacting the precursorcomponent with the reactive component within the bulk of the membrane toform the oxide proton conductor.
 53. A method of making a protonconducting inorganic-organic composite membrane for use inelectrochemical devices, comprising the steps of: (a) precipitating amixture of proton conducting metal oxide particles in suspension andpolymer binder particles in suspension from a solvent; (b) filtering themixed precipitate onto a removable filter element; (c) removing thefiltered mass from the removable filter element; (d) pressing thefiltered mass to form the proton conducting composite membrane.
 54. Themethod of claim 53, wherein electrochemical devices comprise fuel cells.55. The method of claim 53, wherein the solvent comprises water.
 56. Themethod of claim 53, wherein the proton conducting metal oxide ischaracterized in that it possesses easily exchangeable protons, has anopen framework structure with channels, and retains proton conductivityat temperatures in excess of 200° C.
 57. The method of claim 53, whereinthe proton conducting metal oxide comprises insoluble hydrated orhydrous metal oxides selected from the group consisting of hydratedprecious metal containing oxides, acid oxides of the heavy posttransition elements, oxides of the heavier early transition metals,oxide superacids, phosphates of zirconium, phosphates of hafnium,phosphates of titanium, phosphates of lead, phosphates of tin, tungstentrioxide, molybdenum trioxide, hetero-polymolybdates, andhomo-polymolybdates, hetero-polytungstates, homo-polytungstates,hetero-polytantalates, homo-polytantalates, hetero-polyniobates,homo-polyniobates, oxoacids of antimony, and mixtures thereof.
 58. Themethod of claim 57, wherein the hydrated precious metal containingoxides are selected from the group consisting of RuO_(x)(H₂O)_(n) and(Ru-Ti)O_(x) (H₂O)_(n).
 59. The method of claim 57, wherein the acidoxides of the heavy post transition elements are selected from the groupconsisting of acidic antimony oxides and acidic tin oxides.
 60. Themethod of claim 57, wherein the oxides of the heavier early transitionmetals comprise metals selected from the group consisting of molybdenum,tungsten and zirconium.
 61. The method of claim 57, wherein the oxidesuperacids are selected from the group consisting of sulfonatedzirconia, sulfonated titanium oxide, and sulfonated titanium-aluminumoxides.
 62. The method of claim 53, wherein the proton conducting metaloxide comprises a hydrated or hydrous mixed oxide.
 63. The method ofclaim 53, wherein the proton conducting metal oxide comprises elementsforming insoluble hydrated oxides that are not basic.
 64. The method ofclaim 53, wherein the proton conducting metal oxide comprises oxideparticles that form a connected network extending from one face of themembrane to another face of the membrane.
 65. The method of claim 53,wherein the polymer binder is a synthetic organic polymer having amelting point greater than about 300° C.
 66. The method of claim 65,wherein the synthetic organic polymer comprises a fully halogenatedpolymer, a partially halogenated polymer, and mixtures thereof.
 67. Themethod of claim 66, wherein the most preferred halogenated polymers arefluorinated polymers and blends of these polymers.
 68. The method ofclaim 53, wherein the polymer binder comprises perfluorosulphonic acid,polytetrafluoroethylene, perfluroalkoxy derivative of PTFE, polysulfone,polymethylmethacrylate, silicone rubber, sulfonated styrenebutadienecopolymers, polychlorotrifluoroethylene (PCTFE),perfluoroethylenepropylene copolymer (FEP),ethylene-chlorotrifluoroethylene copolymer (ECTFE),polyvinylidenefluoride (PVDF), copolymers of polyvinylidenefluoride withhexa fluoropropene and tetrafluoroethylene, copolymers of ethylene andtetrafluoroethylene (ETFE), polyvinyl chloride, and mixtures thereof.